1. Field of the Invention
The present invention relates to a method for controlling autoignition of a fuel mixture, notably for a diesel type internal-combustion engine, and to an engine using same.
2. Description of the Prior Art
Within the field of engine development, notably diesel type engines, the assignee's constant concern is to reduce the fuel consumption and emissions such as soot and nitrogen oxides (NOx). The assignee has therefore continued work in the field of the ignition combustion mode by compression of a homogeneous feed (referred to as HCCI for Homogeneous Combustion Compression Ignition) whose goal is to significantly reduce the consumption as well as emissions over a wide range of use of these engines, while maintaining their performance.
Thus, for this type of HCCI combustion mode, the conventional combustion by diffusion of fuel conventionally ignited by compression of air or of a mixture of air and of burnt gas is to be replaced by a long-lasting combustion by autoignition of a fuel mixture. This long duration thus allows mixing the fuel coming from an injection nozzle with the gaseous fluid(s) induced into the combustion chamber of the engine, such as air or a mixture of air and of recirculated exhaust gas (EGR), so as to obtain a nearly homogeneous fuel mixture before combustion starts.
Thus, during the progress of the engine compression phase, the fuel mixture reaches a certain temperature and pressure threshold, and combustion of this mixture occurs through autoignition.
This homogeneous type of combustion allows reducing of the fuel consumption and minimizing the production of nitrogen oxides (NOx) and of particles upon combustion.
However, this type of combustion involves quite significant drawbacks.
In fact, to achieve this homogeneous type combustion, it is essential to control the progress of the combustion and timing thereof in the engine cycle. This requires good combustion timing, that is the exact time when the fuel mixture ignites in relation to the engine cycle, and control of the fuel mixture autoignition duration. In fact, autoignition must start when the piston has reached its adequate position, generally in the vicinity of the top dead center (TDC), at the time when the fuel mixture has all the physico-chemical characteristics required for autoignition, and it must end in the vicinity of the bottom dead center of the piston.
It has thus been observed that a shift in start of autoignition, advanced in relation to the position of the piston or retarded in relation to this position, as well as too long or too short a combustion duration, have the effect of generating combustion noise, increased fuel consumption, decreased engine efficiency and emission of a large amount of pollutants such as nitrogen oxides.
Furthermore, the cetane number of the fuel used is one of the parameters to be taken into account for controlling the fuel mixture autoignition process, but combustion timing is dependent on the engine working point, regarding the pressure as well as the temperature of the engine. A compromise over the cetane number is therefore necessary to be able to operate the engine over all the engine speed and load ranges thereof.
The present invention thus aims to overcome the aforementioned drawbacks by providing a fuel mixture autoignition control method allowing the fuel properties to be adjusted to the combustion requirements.